As an image reading apparatus to be applied to a copying machine, a scanner, a facsimile, and so forth, a contact image sensor that scans a document as a reading object with a one-dimensional image pickup device (line sensor) to generate image data corresponding to the document is put into practical use. The contact image sensor includes a plurality of sensor chips arranged linearly in a main-scanning direction, and each of the plurality of sensor chips includes a plurality of image pickup elements arranged linearly in the main-scanning direction at a predetermined arrangement pitch. However, since no image pickup element is disposed between adjacent sensor chips of the plurality of sensor chips, if the arrangement pitch of the image pickup elements is small, there arises a problem in which a loss of data corresponding to a position between the adjacent sensor chips is conspicuous and quality of a read image is degraded.
As a countermeasure for this, proposed is an apparatus in which a plurality of sensor chips are linearly arranged so that the arrangement pitch of image pickup elements between the adjacent sensor chips is twice as large as the arrangement pitch of image pickup elements in each of the sensor chips, and a loss of data in a position between the adjacent sensor chips is interpolated by signal processing (e.g., see Patent Document 1). Patent Document 1 describes an apparatus using, as interpolation data, an average value of data of two pixels at both sides of a data loss position corresponding to the position between the adjacent sensor chips, and also describes an apparatus using, as interpolation data, a value obtained from a calculation made by using a quartic approximate curve derived from data of two pixels at each side (i.e., four pixels in total) of the data loss position corresponding to the adjacent sensor chips.